Chapter 9: MRI Magnetism, Shimming, Gradients & RF Systems

Comprehensive Question and Answer flashcards for Chapter 9 covering magnetism types, magnet systems, shimming, gradient functionality, RF systems, and MRI safety.

What determines the atomic number and chemical element of an atom?

The number of protons in the nucleus.

Why is hydrogen used to form the MRI image?

Hydrogen protons are manipulated to form the MRI image because hydrogen is abundant in body water and fat.

How do diamagnetic materials react in a magnetic field, and what is the electron pairing status?

Diamagnetic materials have paired electrons and are slightly repelled by the magnetic field (Memory: PAIR = PUSH).

If a material possesses both paramagnetic and diamagnetic traits, how is it categorized?

The material will be categorized as paramagnetic.

What characteristics and risks are associated with ferromagnetic materials?

Ferromagnetic materials exhibit strong attraction and alignment, present torque and projectile risks, and may retain magnetism.

What are the main features and typical field strength of permanent MRI magnet systems?

Permanent magnets are usually open, operate at $0.2\text{--}0.3\text{ T}$, are always ON, have low fringe fields, are very heavy, and require low power/maintenance.

What causes heat in resistive MRI magnet systems?

Electrical resistance in the copper-wound solenoids creates heat.

What cryogen is typically used in superconducting MRI magnets, and what is the clinical field strength range?

Superconducting magnets use liquid helium cryogens and typically operate in the $0.5\text{--}3\text{ T}$ clinical range.

What is the 5-gauss line and what is its value in millitesla?

The 5-gauss line is the typical boundary used for MRI Safe vs. unsafe objects, equal to $0.5\text{ mT}$ ($0.5\text{ mT} = 5\text{ G}$).

What is magnetic field homogeneity and in what units is it measured?

Homogeneity refers to how uniform the main magnetic field $B_0$ is, measured in parts per million ($\text{ppm}$).

What are the homogeneity levels for factory magnets, clinical scanners, and clinical spectroscopy?

Factory homogeneity is approximately $100\text{ ppm}$, clinical scanners require approximately $4\text{ ppm}$, and clinical spectroscopy requires better than $1\text{ ppm}$.

What is the key difference between passive shimming and active shimming?

Passive shimming uses steel discs/plates embedded during installation to fix magnet/environment inhomogeneities, while active shimming uses solenoid electromagnets during scanning to correct patient-caused inhomogeneities.

What causes the characteristic knocking noise produced by the MRI machine?

Rapid current changes in the gradient coils switching from positive to negative.

Which gradient coil orientations correspond to the sagittal, coronal, and axial imaging planes?

X gradient = Left to right (Sagittal); Y gradient = Anterior to posterior (Coronal); Z gradient = Head to foot (Axial).

What are the four main applications of magnetic gradients in MRI?

Slice selection, spatial encoding, phase encoding, and frequency encoding.

How does gradient magnetic flux direction relative to $B_0$ alter proton precessional frequency?

If the gradient field flows with $B_0$, main field strength increases and precessional frequency speeds up. If it flows opposite $B_0$, main field strength decreases and precessional frequency slows down.

What is the conversion ratio between $\text{G/cm}$ and $\text{mT/m}$?

$1\text{ G/cm} = 10\text{ mT/m}$.

How are gradient rise time, slew rate, maximum amplitude, and duty cycle defined?

Rise time is the ramp-up time to max amplitude; Maximum amplitude is maximum strength; Slew rate is how quickly gradients change (+ to -); Duty cycle is the percentage of TR period allowed at maximum amplitude.

What safety issues can result from rapid gradient switching?

Peripheral nerve stimulation, including mild cutaneous sensations, muscle contractions, and retinal phosphenes.

What are the two primary responsibilities of the RF system in MRI?

1) Transmit RF energy at hydrogen's resonant frequency to disturb protons, and 2) Receive tiny signals emitted by hydrogen protons to create the image.

Why should RF coil wires never be looped?

Looped wires can induce electric current within a magnetic field.

Why must RF coils be prevented from directly touching the patient's skin?

RF heating can cause cutaneous burns.

How does Faraday's Cage perform RF shielding for an MRI room?

It prevents outside radio frequency interference from ruining image quality by lining room walls, floor, and ceiling with copper sheets, and using copper mesh in the windows.